Actually you can.
I bought another character slot and now have 12/13 active Characters (meaning 12 of 13 possible characters) on my main server, as well as an additional 13th slot on the other servers. Subscribers are limited to 12 slots without buying additional slots.

ETA: You have to claim the item from the Cartel Shop and then use the item in your inventory. This opens the slot. I think I did a restart of the game to make sure it is shown correctly.

^ This.

Some unlocks do bug on characters thus claiming it and not activating it. The only way to get it fixed is a CS ticket.

I think i read in another thread the limit is 18 character slots per server.